Cook onions in a pan over moderate heat until soft and translucent, about 8 minutes
Add garlic and cook for 2 minutes more
Add cinnamon stick, ground spices, coriander stalks and a big pinch of salt, cook for 3 minutes more with a splash of oil to make things sticky
Add drained lentils and top up with fresh water about 2cm higher than ingredients in pan
Bring to boil, skim off any froth, then simmer over moderate heat for around 30 minutes until lentils are tender
Blend soup with coriander leaves and lemon juice
Check seasoning and adjust salt as needed
Serve with a big spoon of Greek yogurt, chilli flakes, sumac, and a glug of olive oil
